help understand strange behavior in a lazy code
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ghc
Does any method exist quickly to detect valid range versions of used library
I'm a beginner Haskell programmer. I have written some useful code for the last six month. And I want to release a library from it. The code will use system installation cabal as any Haskell library. A library is released with cabal has a meta data file where there is a logical predicate from the libraries and their versions. A develope...
Why does s ++ t not lead to a stack overflow for large s?
I'm wondering why Prelude> head $ reverse $ [1..10000000] ++ [99] 99 does not lead to a stack overflow error. The ++ in the prelude seems straight forward and non-tail-recursive: (++) :: [a] -> [a] -> [a] (++) [] ys = ys (++) (x:xs) ys = x : xs ++ ys EDIT: Initially, I thought the issue has something to do with the way ++ is de...
In Haskell, what does it mean if a binding "shadows an existing binding"?
I'm getting a warning from GHC when I compile: Warning: This binding for 'pats' shadows an existing binding in the definition of 'match_ignore_ancs' Here's the function: match_ignore_ancs (TextPat _ c) (Text t) = c t match_ignore_ancs (TextPat _ _) (Element _ _ _) = False match_ignore_ancs (ElemPat _ _ _) (Text t) = False match_ig...
Why can't GHCi resolve the kind of [[]] ?
Why can't Haskell resolve the kind of [[]] (A list of lists)? Why isn't it simply * -> *, as I can give it a type like Int, and get [[Int]], which is of kind *. ...
List of GHC extensions
I wanted to use {-# LANGUAGE OverloadedStrings #-} but I forgot how it's called. This kind of thing isn't hoogle-able, and also it takes some time finding using google*. Is there somewhere a list of GHC extensions named as they are in the LANGUAGE pragma? * My googling search journey: Google Haskell at wikipedia GHC at wikipedia GHC ...
How can I compile a GUI executable with ghc?
I ported a little Haskell program I wrote from Mac to Windows. It's a GUI application (wxHaskell, compiled with ghc 6.12.1), so it does not need the command prompt window to open. It does so, anyway, so my question: What must I do so that the program starts without opening a prompt window first? Is there some ghc switch for this? ...
Curious about the HashTable problem
I read that hash tables in Haskell had performance issues (on the Haskell-Cafe in 2006 and Flying Frog Consultancy's blog in 2009), and since I like Haskell it worried me. That was a year ago, what is the status now (June 2010)? Has the "hash table problem" been fixed in GHC? ...
What does the `forall` keyword in Haskell/GHC do?
I'm beginning to understand how the forall keyword is used in so-called "existential types" like this: data ShowBox = forall s. Show s => SB s This is only a subset, however, of how forall is used and I simply cannot wrap my mind around its use in things like this: runST :: forall a. (forall s. ST s a) -> a Or explaining why these ...
Orphaned instances in Haskell
When compiling my Haskell application with the -Wall option, GHC complains about orphaned instances, for example: Publisher.hs:45:9: Warning: orphan instance: instance ToSElem Result The type class ToSElem is not mine, it's defined by HStringTemplate. Now I know how to fix this (move the instance declaration into the module where...
Categorizing GHC extensions
I was wondering if GHC's extensions can be divided into basically two different categories those that provide "syntactic suggar" or convenience and those that introduce something new, a new paradigm for instance. Now provided one could divide the existing extensions into the above categories which extension would fit into which categ...
how can I build cabal-install on eeePc 701 / Ubuntu Netbook Remix 1.6 (Lucid)
Not a programming question, but the first time I see something like this. UNR 1.6 (based on Ubuntu 10.04) installs the 6.12.1 version of GHC. So, in order to build cabal-install-0.8.2, I have to install the libghc6- packages of parsec, mtl, network and zlib. Then, after launching 'sh ./bootstrap.sh', I get: Checking installed packages ...
GHC's RTS options for garbage collection
I have a Haskell program which processes a text file and builds a Map (with several million elements). The whole thing can run for 2-3 minutes. I found that tweaking the -H and -A options makes a big difference in running time. There is documentation about this functionality of the RTS, but it's a hard read for me since I don't know the...
Memory footprint of Haskell data types
How to find the actual amount of memory required to store a value of some data type in Haskell (mostly with GHC)? Is it possible to evaluate it in runtime (e.g. in GHCi) or is it possible to estimate memory requirements of a compound data type from its components? In general, if memory requirements of types a and b are known, what is me...
Merging/union two classes into one in Haskell
I have two non-overlapping sets of types and want to make other set which is union of these two. Code sample: class A a class B b class AB ab instance A a => AB a instance B b => AB b GHC 6.12.3 doesn't allow to declare this with error message: Duplicate instance declarations: instance (A a) => AB a -- Defined at playgro...
Installing a Haskell package from Hackage through Cabal gets stuck at dependency resolution
Here is the output from verbose mode. I know this used to work for me before but now this happens. I've tried it with other packages and got the same result. $ sudo cabal install test-framework-quickcheck2 -v /usr/local/bin/ghc --numeric-version looking for package tool: ghc-pkg near compiler in /usr/local/bin found package tool in /u...
Haskell: print TextEncoding
Haskell newbie here. $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 While trying to debug weird locale-related bug in third-party Haskell program, I'm trying to print default encoding: import System.IO main = do print localeEncoding But it fails: $ ghc -o printlocale main.hs main.hs:4:2: No...
Haskell Error: parse error on input `='
Specs GHC 6.12.1 Mac OS X 10.6.4 x64 MacBook Pro Problem I'm having trouble using let syntax. The following code refuses to compile: module Main where main = let x = 1 y = 2 z = 3 in putStrLn $ "X = " ++ show x ++ "\nY = " ++ show y ++ "\nZ = " ++ show z I tried tabbing in y = 2 and z = 3 even more. No dice. ...
How to get only the object file without main function by ghc?
[Source Code] data FooBar = Foo | Bar [Command] $ ghc -c foo_bar.hs foo_bar.hs:1:0: The function 'main' is not defined in module 'Main' [Configuration] Glasgow Haskell Compiler, Version 6.12.3, for Haskell 98, stage 2 booted by GHC version 6.10.4 ...
cross-compiling haskell code through ghc and mingw tools
I've tried -fvia-C and the -pgms, but none of them manage to create an executable, splurting out lots of errors like Warning: retaining unknown function ``L4' in output from C compiler. ...